The invention relates to a mixture which can be polymerized by radiation, in particular ultraviolet or visible light, and which contains a water-insoluble binder which is soluble in aqueous alkaline solutions, a free-radical-polymerizable compound and a radiation-activable polymerization initiator for the polymerizable compound, and also to a copying material having a copying layer composed of the said mixture.
Radiation-polymerizable and photopolymerizable mixtures of the class specified are known. They contain, for example, copolymers of acrylic acid or methacrylic acid, of maleic acid, crotonic acid or itaconic acid with neutral comonomers, for example esters of (meth)acrylic acid, styrene, vinyl acetate or (meth)acrylonitrile, as alkali-soluble binders. Such mixtures are described, inter alia, in DE-A-2,064,080 and 2,205,146. In general they are used for the production of letterpress and lithographic printing plates and of photoresists, in particular those which are processed by the dry resist process.
For the production of lithographic printing plates it is important that the radiation-cured or light-cured coating have a high abrasion resistance in order to yield high print runs. This property is primarily determined by the polymeric binder. For lithographic printing plates based on negative working diazonium compounds it is known, for example, from DE-A-2,739,774 that printing forms with high run performance are obtained with polyurethanes as binders. Photopolymerizable mixtures containing such polymers as binders are also described. The disadvantage of such coatings is the poor solubility in aqueous solutions, a high proportion of solvent therefore being necessary in the developer for complete development, as described in EP-A-0,072,918 or DE-A-2,404,239. The incorporation of quaternary and quaternizable nitrogen atoms in a high-molecular weight thermoplastic polyurethane containing lateral unsaturated, photopolymerizable double bonds makes it possible to obtain, as described in EP-A-0,054,150, photopolymerizable copying materials which can be developed with weakly acidic aqueous solutions. The disadvantage of the printing layers which contain such binders is the poor resistance to the acidic fountain solutions used in conventional offset printing since polyurethanes containing basic nitrogen atoms are soluble, or at least extremely swellable, in an acidic medium. Much more advantageous are photopolymerizable layers, which can be developed in an alkaline developer without solvent added.
The Japanese Application JP-A-54/14491 (C.A 92, 32017b) describes the reaction of polyesters containing carboxyl groups with monofunctional, unsaturated isocyanates. The number of urethane groups per molecule which impart good properties such as abrasion resistance to the system is in this case very limited.
DE-A-3,107,585 reveals the use, for the production of printing plates, of unsaturated polyurethanes which contain carboxyl groups and which are produced by reacting an NCO-terminated prepolymer with an olefinically unsaturated diol and then reacting the OH groups still left with a cyclic carboxylic acid anhydride. GB-A-2,091,278 describes the use of polyurethanes containing a carboxylic acid group at one end and an olefinically unsaturated group at the other for the production of photoresists. The disadvantages of the polymers specified in these publications is the low content of carboxylic acid groups per molecule, which manifests itself at higher molecular weights in a poor alkali solubility of the layers.
DE-A-3,639,757 reveals polyurethanes containing carboxyl groups which are used in photosensitive mixtures in combination with diazonium salts. DE-A-3,716,607 describes polyurethanes which are derived from these polymers and which contain, in addition to the carboxyl groups, unsaturated groupings which are photo-crosslinkable. These polymers can be used in printing plates alone or in combination with monomers or diazonium salts. To develop the printing layers, a developer is used which contains 3% benzyl alcohol.
JP-A-61/20939 (C.A. 105, 52221d) describes polyurethanes containing carboxyl groups which are produced by reacting a prepolymer prepared from a polypropylene glycol and an excess of tolylene diisocyanate, with the monoamide produced from diethanolamine and succinic anhydride. After the preparation, the carboxylic acid groups are neutralized with dimethylamine and during this process terminal NCO groups still present are deactivated. The resins are used in printing plates in combination with a methacrylic acid/methacrylate copolymer, a monomeric polymerizable acrylate, a photoinitiator and a dyestuff as photosensitive layer.
The important disadvantage of this binder is the long reaction time associated with its synthesis. Since carboxylic acid groups which render the solution acidic are present during the preparation of the polyurethanes, the reaction rate of the NCO groups with OH groups is considerably reduced (cf. G. W. Becker and D. Braun, Kunststoffhandbuch (Plastics Manual), volume 7, Carl Hanser Verlag, Munich 1983). As the reaction proceeds, the reaction rate decreases further. However, as extensive a reaction as possible is necessary to achieve a high molecular weight. At the same time, this can only be achieved by extremely long reaction times. Reducing the reaction time (prematurely interrupting the reaction by means of dimethylamine) results in short-chain polyurethanes which yield printing plates with poor mechanical properties; in addition, the layers are often sticky in the unexposed state.
U.S. Pat. Nos. 4,183,836 and 4,203,883 reveal the preparation of stable aqueous anionic polyurethane dispersions for coating textile materials via a prepolymer. For this purpose, a NCO-terminated prepolymer containing carboxyl groups is first synthesized; the acid groups are then neutralized in an aqueous solution of a tertiary amine and the NCO groups are reacted with a diamine as chain extender. The process is limited to certain, generally aliphatic diisocyanates which only react slowly with water. No mention is made of any suitability as a binder for photosensitive mixtures.